1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus and, more particularly, to an information recording/reproducing apparatus for reproducing information from a recording medium by using the scanning tunneling electron microscope technology.
2. Related Background Art
The memory material technology is the core of the field of electronics industries, such as computers, peripheral devices of computers, video discs, and digital audio discs, and researchers are very enthusiastically developing memory materials in recent years. Conventionally, magnetic memories or semiconductor memories using magnetic substances or semiconductors as their materials have been the mainstream of memories. With recent development in the laser technology, however, optical memories using thin organic films consisting of, e.g., organic dyes or photopolymers have become available. The optical memory is an inexpensive recording medium with a high recording density.
Meanwhile, a scanning tunneling microscope (to be abbreviated as an STM hereinafter) capable of directly observing the electron structure of a surface atom of a conductor has been developed [G. Binning et al., Phys. Rev. Lett., 49, 57 (1982)], and this makes it possible to measure real space images of samples with a high resolving power regardless of whether the samples are single-crystal or amorphous. The STM also has an advantage that it can observe samples at a low electric power without damaging them with an electric current and can be operated even in the atmosphere. Therefore, the STM is expected to be put to use in a wide variety of applications.
The STM makes use of a phenomenon in which a tunnel current flows between a metal probe (probe electrode) and a conductive substance, as a sample, when the probe and the sample are moved closer together to a distance of about 1 nm with a voltage applied between them. Since this tunnel current responds exponentially to the change in distance between the probe and the sample, the STM is very sensitive to the surface condition of the sample. In addition, by scanning the probe in such a manner as to maintain the tunnel current constant, various pieces of information concerning the entire electron cloud in a real space can be read. The resolving power in the direction of the surface in this case is approximately 0.1 nm. Therefore, the application of the principle of the STM readily enables high-density recording and reproduction on the order of atoms (subnanometer).
As an example, a method of performing recording and reproduction by using the STM has been proposed, in which a thin film layer consisting of a material having a memory effect in voltage-to-current switching characteristics, such as a conjugated .pi. electron-based organic compound or a chalcogen compound, is used as a recording layer (Japanese Laid-Open Patent Application Nos. 63-161552 and 63-161553). That is, by applying a voltage exceeding a threshold value to this recording layer, the recording layer becomes transient between two different states according to the polarity of the voltage applied, and these states persist stably when no voltage is applied. The state of the recording layer can be detected by the value of a tunnel current obtained when the distance between the surface of the recording layer and a probe is maintained constant. Since the voltage exceeding the threshold value can be applied to the recording layer by using the probe for detecting the tunnel current, recording and reproduction of binary information can be performed at a recording density corresponding to the surface resolving power of the STM. According to this method, assuming the size of a recording bit is 10 nm, large-capacity recording and reproduction of up to 10.sup.12 bits/cm.sup.2 can be performed. FIG. 1 shows an example of the construction of an information recording/reproducing apparatus according to this method.
In this information recording/reproducing apparatus, scanning of a probe electrode (probe) 102 for tunnel current detection is effected in each of the X, Y, and Z directions by a cylindrical piezoactuator 101. The X and Y directions are directions along the surface of a recording medium 103, and the Z direction is a direction perpendicular to the surface of the recording medium 103. A tunnel current detected by the probe electrode 102 is amplified by a current amplifier 104, and a high-frequency component of the current is extracted by a high-pass filter (HPF) 106 and output as reproducing information. A low-frequency component of the output from the current amplifier 104, on the other hand, is extracted by a low-pass filter (LPF) 105 and phase-compensated by a phase compensating circuit 107. An output from the phase compensating circuit 107 is supplied to an error amplifier 108 and fed back to the piezoactuator 101 via a sample-and-hold circuit 109. This feedback loop is provided to maintain the distance between the probe electrode 102 and the recording medium 103 constant by keeping the tunnel current constant. The distance can be controlled by an offset voltage to be applied to the error amplifier 108.
Reproduction of information performed by the above information recording/reproducing apparatus will be described below. Assume that the surface of the recording medium 103 is flat even on the atomic level and dot-like recording bits are formed in a direction of the surface of the recording medium 103. Assume also that the scanning of the probe electrode 102 is effected in the direction of the surface of the recording medium 103 while the distance between the surface of the recording medium 103 and the tip of the probe electrode 102 is precisely maintained constant. The consequent tunnel current changes, ideally, between two large and small values in accordance with the recording condition, as shown in FIG. 2. A high-frequency component of the tunnel current exhibits, as shown in FIG. 3, a change represented by a pulse that is oscillated upward or downward for each change in tunnel current shown in FIG. 2. That is, in the high-frequency component of the tunnel current, a pair of two pulses (one on the (+) side, and the other on the (-) side) correspond to one recording bit. Reproduction of information is performed by detecting the high-frequency component of the tunnel current.
Suppose a step structure (atomic step) with a thickness of about an atomic layer exists on the surface of the recording medium 103. In this case, this step of the atomic order unavoidably remains on the surface of the recording medium even if a thin gold film, which is formed by a molecular beam epitaxial (MBE) process and smooth on the atomic order, is used as the undercoating electrode of the recording medium. The consequent tunnel current also changes in the location of the step, as shown in FIG. 4, and so the high-frequency component of the tunnel current contains a pulse component even in the location of the step, as shown in FIG. 5.
The above conventional information recording/reproducing apparatus has a problem that if a step is present on the surface of a recording medium, a signal is also mixed from this step into the high-frequency component of a tunnel current, adversely affecting reproduction of information.